Timeslot Sharing Using Unbalanced QPSK Modulation

ABSTRACT

In a radio transmission scheme reusing slots such as by using Orthogonal Sub Channels (OSC) in the downlink channel a parameterized, hybrid quaternary modulation is employed. In the hybrid quadrature modulation, the symbol constellation is in quadrature, with the 4 symbols lying on the unit circle in the complex plane. The orthogonality of the 1 and Q branches is preserved. A cross power branch ratio parameter α is introduced, allowing the total energy of the signal to be divided unequally between the two sub channels. This parameter α may be changed from over time.

TECHNICAL FIELD

The present invention relates to a method and a device for modulating data. In particular the present invention relates to modulation of data transmitted to two mobile stations is a cellular radio system simultaneously on shared channel.

BACKGROUND

The concept of Orthogonal Sub Channels (OSC) proposed in “Voice capacity evolution with orthogonal sub channel, see” 3GPP TSG GERAN Meeting #33, GP-070214 has been well accepted. One reason is that the dramatic growth of the subscriber base in developing countries imposes a tremendous pressure on the Base Transceiver Station (BTS) hardware resources. Therefore, a study item for a technique dubbed MUROS (Multiple User Reusing One Slot) has been opened in the standardization of GSM, see “New Study Item on Multi-User Reusing One Slot (MUROS” 3GPP TSG GERAN Meeting #36. GP072027.

OSC is a multiplexing technique that allows two users to share the same frequency and time slot. It relies on Quadrature Phase Shift Keying (QPSK) modulation in the downlink channel. The I and Q branches of a modulated signal form two sub channels. The data carried by the I branch belongs to a first user, while the data carried by the Q branch belongs to a second user. Orthogonality is preserved by using a root raised cosine pulse shaping filter with a bandwidth equal to the reciprocal of the symbol period. At the receiver side, the mobile stations (MS) rely on orthogonal training sequences in order to separate the sub channels, see 3GPP TSG GERAN Meeting #33, GP-070214. In the uplink channel, the two mobile stations sharing the same channel also transmit in the same frequency and time slot.

The base station separates the two users using a multi-user detector, e.g. successive interference cancellation.

It has been stated in “New Study Item on Multi-User Reusing One Slot (MUROS” 3GPP TSG GERAN Meeting #36. GP072027, that the physical layer for MUROS must support legacy mobile stations in one of the sub channels. However, it has been reported, see e.g. “The Performance of OSC and Feasibility Analysis”, 3GPP TSG GERAN Meeting #36. GP071663 and “Discussion Paper on OSC”, 3GPP TSG GERAN Meeting #36, GP071785, that OSC may not be backward compatible with legacy Gaussian minimum shift keying (GMSK) mobile stations. The problem arises in the downlink channel, because a legacy receiver exhibits very poor performance when the transmitted signal is Quadrature phase-shift keying (QPSK) modulated.

Moreover, some concerns have been raised about the constraints that OSC imposes on power control and the need to subdivide the cells of a cellular radio system, leading to additional handovers and hence presenting a potential to degrade some of the Key Performance Indicators (KPI) such as dropped calls, see also “On Orthogonal Sub channels”, 3GPP TSG GERAN Meeting #36, GP071720. Also so far, none of the proposals for MUROS as set out in Voice capacity evolution with orthogonal sub channel, “3GPP TSG GERAN Meeting #33. GP-070214 and “Speech capacity enhancements using DARP”, 3GPP TSG GERAN Meeting #36. GP071739 fulfills the backward compatibility requirement for legacy mobiles.

Hence, there exist a need for a method and a system that eliminates or at least reduces the negative aspects of introducing OSC in existing cellular radio systems.

SUMMARY

It is an object of the present invention to overcome or at least reduce some of the problems associated with the introduction reuse of slots, such as by using OSC, in cellular radio systems.

It is another object of the present invention to provide a method and a device that is capable of improving the transmission of data in a cellular radio system employing USC.

These objects and others are obtained by the method, a modulator a mobile station and cellular radio system as set out in the appended claims. Thus, by modulating data transmitted using a QPSK modulation scheme in a cellular radio system to two mobile stations multiplexed on a shared channel comprising two branches, such that the total energy of the QPSK modulated is divided unequally between the two branches of the modulated signal an improved radio system can be obtained.

Hence, in the downlink channel a parameterized, hybrid quaternary modulation is employed. In the hybrid quadrature modulation, the symbol constellation is in quadrature, with the 4 symbols lying on the unit circle in the complex plane. The orthogonality of the I and Q branches is preserved. However, a cross power branch ratio parameter α is introduced, allowing the total energy of the signal to be divided unequally between the two sub channels. This parameter α may be changed from over time. The parameter may for example be changed from one transmission time slot to the next transmission time slot. It is chosen so that 0≦α≦1. In the extreme case when α=1 the power is divided equally between the I/Q branches, resulting in ordinary QPSK modulation. When α=0 all the signal power is given to one of the branches yielding BPSK modulation. Other values of a causes intermediate distributions of the total energy between the I and Q sub channels. In accordance with one embodiment, the parameter α can be chosen adaptively, for example based upon feedback from one or both mobile stations receiving data via the shared downlink channel, or using a fixed scheme.

The invention also extends to a cellular radio system, a modulator and a mobile station adapted to make use of the method in accordance with the above.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail by way of non-limiting examples and with reference to the accompanying drawings, in which:

FIG. 1 is a flow chart illustrating different steps performed when modulating data,

FIG. 2 is a view of a signal constellation,

FIG. 3 is a view illustrating the cross power ratio between the I and Q branches,

FIG. 4 is a view of a modulator.

FIG. 5 is a view of a cellular radio system employing OSC.

FIGS. 6 a and 6 b illustrate different procedural steps performed in a receiver receiving data modulated using OSC.

DETAILED DESCRIPTION

In FIG. 1 a flowchart illustrating different steps performed when modeling data in accordance with one embodiment of the present invention is shown. First in a step 101 data is to be transmitted to different users of a cellular radio system are multiplexed on Orthogonal Sub Channels (OSC). In accordance with one embodiment of the present invention, an ordinary QPSK constellation consisting of the following points may be chosen.

$\begin{matrix} {\sqrt{\frac{1}{2}} + {j\sqrt{\frac{1}{2}}}} & {\sqrt{\frac{1}{2}} - {j\sqrt{\frac{1}{2}}}} & {{- \sqrt{\frac{1}{2}}} + {j\sqrt{\frac{1}{2}}}} & {{- \sqrt{\frac{1}{2}}} - {j\sqrt{\frac{1}{2}}}} \end{matrix}$

Using the ordinary QPSK constellation as a starting point, a cross power ratio parameter 0≦α≦1 is then chosen based on for example, a predefined criterion or on feedback from one or many mobile stations in a step 103. A new quadrature constellation is then created as follows in a step 105:

$\begin{matrix} {{\alpha \sqrt{\frac{1}{2}}} + {j\sqrt{2 - \alpha^{2}}\sqrt{\frac{1}{2}}}} & {{\alpha \sqrt{\frac{1}{2}}} - {j\sqrt{2 - \alpha^{2}}\sqrt{\frac{1}{2}}}} & {{{- \alpha}\sqrt{\frac{1}{2}}} + {j\sqrt{2 - \alpha^{2}}\sqrt{\frac{1}{2}}}} & {{{- \alpha}\sqrt{\frac{1}{2}}} - {j\sqrt{2 - \alpha^{2}}\sqrt{\frac{1}{2}}}} \end{matrix}$

In the following a quadrature constellation such as the one above will be referred to as adaptive α-QPSK constellation. Next in a step 107 data are transmitted multiplexed to the users using the modulation determined in step 105.

In FIG. 2, the four points in an adaptive α-QPSK constellation are shown, with α=0.6.

In an adaptive α-QPSK constellation, the cross power ratio between the I and Q branches is

$\begin{matrix} {\chi = \frac{\alpha^{2}}{2 - \alpha^{2}}} & (1) \end{matrix}$

In FIG. 3 the cross power ratio x between the I and Q branches is shown as a function of a.

For example, if α=0.6 then the power of the I branch is approximately 6.6 dB lower than the power of the Q branch. It is preferred to keep the total energy in the symbol constellation constant, independently of the value of α.

In FIG. 4 an exemplary modulator 400 used in transmission of data using an α-QPSK modulation is depicted. The modulator 400 comprises initial modulators 401 and 403 for receiving and QPSK modulating data sequences to be transmitted to two different mobile stations. The modulators 401 and 403 are coupled to a mapping unit 405 adapted to map the QPSK signals from the modulators 401 and 403 in accordance with an adaptive α-QPSK constellation such as the one described hereinabove. The adaptive α-QPSK constellation sequence formed in the unit 405 is forwarded to a rotation block 407 connected to a pulse shaping filter 409 which in turn is connected to a unit 411 adapted to amplify and mix-up the multiplexed data sequence to be transmitted to the intended receivers to the carrier frequency.

The modulator 400 may receive feedback from one or both mobile stations to which data is transmitted. In response to received feedback the modulator can be adapted to adjust a accordingly. For example a may be set to depend upon the distances from the two mobile stations to the Base Transceiver Station (BTS), the reported received signal quality (RXQUAL), or upon the capabilities of the mobile stations (e.g. legacy/OSC aware).

In FIG. 5 an exemplary process of a cellular radio system 500 is illustrated. The system 500 comprises a BTS receiver 501 for receiving data transmitted from a number of mobile stations 503 and 505 connected to the cellular radio system via the Base Transceiver Station 501. The mobile stations 503 and 505 may be OSC aware or non-OSC aware. In the example depicted in FIG. 5, the mobile station 503 is OSC aware whereas the mobile station 505 is non-OSC aware. The system 500 further comprises a modulator 507, such as the modulator in accordance described above in conjunction with FIG. 4 for generating an adaptive α-QPSK modulated signal. In addition the system comprises a control unit 509 for calculating a suitable value α and for feed the α-value to the α-QPSK modulator. The value of α may change from one transmission interval to a subsequent transmission interval. It is also possible to use a constant, predefined value of α.

An OSC-aware receiver, such as Mobile station 503 in, FIG. 5 will exhibit better performance if the value of α (alternatively χ) is used during synchronization/channel estimation and demodulation. If it is not known, then it can be estimated.

In FIGS. 6 a and 6 b different steps performed in a receiver receiving data modulated in accordance with the above are shown. In FIG. 6 a steps performed in a receiver 600 during synchronization/channel estimation and demodulation if α is not known, First in a block 601, the training sequences for users multiplexed on the OSC sub-channels are obtained together with the sampled received signal. Next, in a block 603, the synchronization position, channel impulse response and cross power branch ration parameter α or χ, or any other parameter correlated to α, are estimated thereby providing an estimate on the α-QPSK constellation used by the modulator in the transmitter. The estimation in block 603 may be performed jointly. The estimated α-QPSK constellation of block 603 is then used in a block 605 when demodulating the received signal.

In FIG. 6 b steps performed in a receiver 650 during synchronization/channel estimation and demodulation if α is known, First in a block 651, the training sequences for users multiplexed on the OSC sub-channels are obtained together with the sampled received signal and the cross power branch ration parameter α or χ or any other parameter correlated to α are obtained. Next, in a block 653, the cross power branch ration parameter, such as α or χ are used during estimation of the synchronization position and channel impulse response. The α-QPSK constellation is then used in a block 655 when demodulating the received signal.

Using adaptive α-QPSK modulation as described herein will improve the performance of a legacy receiver, while moderately punishing an OSC-aware receiver. The performance results for 8PSK-quarter rate together simulations indicate that OSC enhanced with adaptive α-QPSK modulation is a competitive alternative for a quarter rate speech bearer, even when a legacy mobile occupies one of the sub channels.

Furthermore, using the modulation scheme and modulator as described herein enhances the OSC concept, making it possible to introduce legacy mobiles in one of the sub channels, which is a key requirement for any multiplexing concept complying with MUROS, in addition it allows more flexible power control in OSC. This will help obtain better system performance and will help maintain or improve the KPI's. This is another issue of great importance. The modulation technique is also straightforward to implement in any BTS hardware capable of transmitting 8PSK. 

1-11. (canceled)
 12. A method of modulating data transmitted using a quadrature phase shift keying (QPSK) modulation scheme in a cellular radio system comprising transmitting data from a Base Transceiver Station (BTS) to two mobile stations multiplexed on a shared channel comprising two branches; wherein the total energy of the QPSK modulated signal is divided unequally between the two branches of the modulated signal.
 13. The method of claim 12 wherein the total energy of the QPSK modulated signal is divided unequally in a ratio depending upon a factor selected from the group consisting of the distances of the two mobile stations from the Base Transceiver Station (BTS), the reported received signal quality (RXQUAL) of the two mobile stations, and the capabilities of the mobile stations with respect to the modulation technique.
 14. The method of claim 12 wherein the total energy of the QPSK modulated signal is divided adaptively between the two branches.
 15. The method of claim 14 wherein the energy distribution between the two branches of the QPSK modulated signal is changed from one transmission slot to the next transmission slot.
 16. The method of claim 12 wherein the two mobile stations are Global System for Mobile communication (GSM) mobile stations sharing the same channel using Orthogonal Sub Channels.
 17. The method of claim 12 wherein the QPSK modulated symbols are all located on the unit circle in the complex plane.
 18. A modulator operative to modulate data transmitted as a modulated signal using a quadrature phase shift keying (QPSK) modulation scheme having two branches, comprising: a divider operative to divide the total energy of the QPSK modulated signal unequally between the two branches of the modulated signal.
 19. The modulator of claim 18 wherein the divider is operative to divide the total energy of the QPSK modulated signal unequally in a ratio depending upon a factor selected from the group consisting of the distances of the two mobile stations from the Base Transceiver Station (BTS), the reported received signal quality (RXQUAL) of the two mobile stations, and the capabilities of the mobile stations with respect to the modulation technique.
 20. The modulator of claim 18 wherein the divider is operative to divide the total energy of the QPSK modulated signal adaptively between the two branches.
 21. The modulator of claim 20 wherein the divider is operative to change the energy distribution between the two branches from one transmission slot to the next transmission slot.
 22. The modulator of claim 18 wherein the modulator is operative to locate all QPSK modulated symbols on a unit circle in the complex plane. 